1. Field of the Invention
The present invention relates to an encapsulated contact material and a manufacturing method therefor, and a manufacturing method and a using method for an encapsulated contact, and more specifically, to an encapsulated contact material subject to less variations in contact resistance during switching operation, enjoying satisfactory working life performance, and capable of low-cost production.
2. Prior Art
An encapsulated contact which is used for a reed switch or the like is constructed so that an encapsulated contact material, along with an N.sub.2 gas, for example, is encapsulated in a sealed container which is formed of glass or the like.
In popular conventional encapsulated contact materials, a contact substrate is formed of, e.g., Fe--Ni alloy, and its surface is coated with Rh or Ru, which serves as a contact coating layer. Rh, Ru, etc. are frequently used because they are high-hardness, high-melting metals which have good electrical conductivity and wear resistance.
These conventional encapsulated contact materials are manufactured in a manner such that an intermediate layer is first formed on the surface of the contact substrate by, for example, electroplating the substrate surface with a metal, such as Ag, Au or Cu, and a contact coating layer is then formed on the intermediate layer by plating it with Rh or Ru. The intermediate layer is intended for improved adhesion between the contact substrate and the contact coating layer and prevention of diffusion of Rh or Ru of the contact coating layer into the contact substrate during contact switching operation.
Using Rh or Ru which is an expensive metal, however, the above encapsulated contact materials entail high material cost, thus involving a problem of economic efficiency.
Recently, therefore, there have been proposed low-cost encapsulated contact materials which, like the conventional ones, use Fe-Ni alloy or the like for the contact substrate, and employ a high-melting metal, such as Mo, W, or its alloy, for the contact coating layer.
The contact coating layers of aforesaid encapsulated contact materials have advantageous characteristics such as a high melting point, high hardness, and high electrical conductivity, among other essential characteristics for the contact coating layer. However, the materials of this type have been found to behave in the following manner.
In the case of a material whose contact coating layer is formed of W, for example, a contact working test based on repeated switching operation at 10 Hz may reveal substantial variations in contact resistance or frequent generation of intensive arc discharge in the contact coating layer. If the encapsulated contact material is subject to increased variations in contact resistance, the contact resistance of the encapsulated contact during the switching operation is liable to fluctuate, and besides, heat release from the the encapsulated contact increases. As a result, the working life of the encapsulated contact is shortened and varies substantially, so that the reliability of the contact in actual use is lowered.
These problems are believed to arise because the contact coating layer which is formed of Mo, W, or its alloy does not enjoy satisfactory wear resistance, and lowers the arc characteristics of the contact. Another cause lies in that Mo, W, and their alloys are all susceptible to oxidation in the open air, so that an electrically insulating oxide film is easily formed on the surface of the metal.
In some cases, the oxide film will have already been formed on the surface of the contact coating layer (Mo or W) of the aforesaid contact material by the time the material is handled in the open air before it is encapsulated in the sealed container. Moreover, when the seal area surface of a contact substrate end portion is oxidized before the encapsulation, the contact coating layer may possibly be oxidized simultaneously to form an oxide film on the surface corresponding to the aforesaid contact substrate end portion.
Microscopically, the oxide film has a structure such that oxide particles are distributed in the surface of the contact coating layer. When the encapsulated contact, having the encapsulated contact material sealed therein with its surface in this state, is subjected to a repeated switching operation, the oxide particles migrate or move, and concentrate in the area where they are microscopically in actual contact with one another. Thus, the material which has the oxide film formed on its contact coating layer is supposed to be worsened in the aforementioned working life characteristics.
Normally, the encapsulated contact undergoes the switching operation with a voltage (current) applied thereto.
In general, however, snapping may possibly be caused on the load side during use of electrical equipment. In such a case, the switching operation of the encapsulated contact advances without the application of any voltage (current). Even if snapping is caused by the exhaustion of a light emitting diode or the like which is connected to the encapsulated contact, for example, the contact is subjected to repeated no-load switching operation.
In the case of a reed switch, in particular, its switching magnet operates even in a no-load state, so that there is a high possibility of its encapsulated contact being forced to undergo the no-load switching operation.
In the case of an encapsulated contact having the encapsulated contact material therein whose contact coating layer is formed of Mo, W, or its alloy, the repeated no-load switching operation causes the contact resistance to increase, thereby lowering the stability and reliability of the resulting switch. The aforementioned problems are liable to arise especially in the case where an oxide film is formed on the surface of the contact coating layer of the encapsulated contact material.
In order to solve the above-described problems of the encapsulated contact material whose contact coating layer is formed of Mo, W, or its alloy, the inventors hereof developed and filed an application (Jpn. Pat. Appln. Publication No. 4-19885) for an encapsulated contact material in which a contact coating layer is formed by coating the surface of a contact substrate with a material consisting mainly of Mo, W, Re, Nb, or Ta, and an oxidation-retardant, electrically conductive thin layer of Ru, Rh, Pd, Os, Ir, Pt, Ag, or Au is formed on the coating layer.
In the case of this encapsulated contact material, the oxidation-retardant, conductive thin layer on the surface of the contact coating layer lessens the possibility of the formation of an oxide film which may otherwise be caused when the material is encapsulated in the sealed container. Thus, the encapsulated contact material of this kind is subject to less variations in its initial contact resistance.
Despite the limited variations in the initial contact resistance, however, the encapsulated contact material described above cannot always enjoy good weld resistance and satisfactory arc resistance, in consideration of the requirement for a prolonged working life after initial operation. Accordingly, those characteristics of the contact material are expected to be improved further. To cope with this requirement, the inventors hereof developed and filed an application (Jpn. Pat. Appln. Publication No. 6-39114) for an encapsulated contact material in which a contact coating layer is formed by coating the surface of a contact substrate with a material composed of a matrix which is formed of at least one high-melting metal selected from a group including Mo, Zr, Nb, Hf, Ta, and W, and is loaded with at least one element selected from a group including Li, K, Ce, Cs, Ba, Sr, Ca, Na, Y, La, Sc, Th, and Rb or an oxide thereof, and an encapsulated contact material in which the contact coating layer is loaded with trace amounts of elements, such as Mg, Pb, Sn, Zn, Bi, Ag, Cd, Al, Si, Zr, Ti, Co, Ta, Fe, Mn, Cr, etc.
In the cases of these encapsulated contact materials, the elements, including Li, K, Ce, Cs, Ba, Sr, Ca, Na, Y, La, Sc, Th, Rb, etc., which are contained in the matrix of the contact coating layer have small work functions. In the contact coating layer loaded with these elements, generation of an arc during the switching operation of the encapsulated contact is macroscopically uniform, so that exposure of the contact substrate at the lower part of the coating layer is retarded. Thus, the working life of the material is lengthened.
Microscopically, however, the arc causes infinitesimal indentations to be formed all over the surface of the contact coating layer, and these indentations may change the area of contact between contact coating layers or bite each other, thereby bringing about switching failure (locking). Thus, the working life of the material may possibly be shortened.
In the case of the contact coating layer which further contains the trace elements, including Mg, Pb, Sn, Zn, Bi, Ag, Cd, Al, Si, Zr, Ti, Co, Ta, Fe, Mn, Cr, etc., the trace elements are alloyed with the additive elements, such as Li, K, Ce, Cs, Ba, Sr, Ca, Na, Y, La, Sc, Th, Rb, etc., thereby restraining evaporation of the additive elements and the like. Although this behavior ensures the effect to reduce variations in contact resistance during the switching operation of the encapsulated contact, the working life performance cannot be expected to be much better than that of the material which contains none of the trace elements. In the case of an encapsulated contact which incorporates the encapsulated contact material having its contact coating layer loaded with the trace elements, moreover, there is a problem that variations in working life performance of the encapsulated contacts produced in various production lots are substantial, that is, the stability in product quality is poor.